The present invention relates generally to magnetic resonance imaging systems and more particularly to a method for reducing spin-lattice relaxation time of silicone fluids used in magnetic resonance imaging.
The invention is suitable for use with Nuclear Magnetic Resonance (NMR) techniques, and in particular, although not exclusively, with Magnetic Resonance Imaging (MRI) techniques, but may also be used in Nuclear Quadruple Resonance, Electron Spin Resonance, and other such techniques.
NMR is a technique in which a radio frequency (RF) excitation pulse is applied to a sample in the presence of a magnetic field, and echoes or other signals resulting from the excitation of atomic nuclei with net magnetic moments in the sample are subsequently detected. Hydrogen, whose nuclei are essentially single protons, are the nuclei most commonly probed by MR due to their high abundance in water and fat and their relatively high receptivity (signal amplitude) to the MR experiment. In MRI, the magnitudes of the echo signals are used to construct an image. The concentration of hydrogen atoms will generally correspond to the water and/or fat density in the sample. MRI is therefore widely used for imaging tissue in the human body.
The echo signals will have various parameters associated with them, such as the spin-lattice relaxation time, T1 and the spin-spin relaxation time, T2. These parameters are properties that are inherent to the material being sampled. The relaxation parameters T1 and T2 are often used to provide contrast in images, because the typical variation in these parameters may be much larger than that in straightforward proton density. For example, the difference in the proton concentration between normal and abnormal tissue in the human body may only be a few percent, while the difference between the relaxation parameters may be much higher. Grey and white areas in the brain also have very different T1 and T2 values.
It is common practice in the industry to utilize phantoms to calibrate the MRI machines between uses. The phantoms are generally shatterproof tanks which contain a liquid filler material. The phantom is positioned inside a head coil on top of a gray foam positioner. The positioner is mounted onto the patient bed that slides in and out of the magnetic bore. The shape of the tank and the composition of the filler material are specifically calibrated to produce a specific pattern of MR signals which are used to ensure the accuracy of the MRI machine.
Poly(dimethylsiloxane) fluid, commonly called silicone oil or silicone fluid, is a very useful non-toxic phantom filler for magnetic resonance. These fluids are non-electrically conductive and have a low dielectric constant which greatly reduces interferences with the RF fields emitted by the MRI system""s RF coils compared to water-based phantom filler materials. Silicone fluids also have self-diffusion characteristics advantageous for calibrating diffusion weighted imaging sequences.
A key disadvantage with the use of silicone oils in MR phantoms is in its excessively long T1 time. This is especially disadvantageous when used in MRI systems operating at main magnetic fields of 1 Tesla or greater.
The present invention proposes dissolving a relatively small amount of gadolinium beta-diketonate (a common metallocomplex) in the silicone oil to greatly reduce the spin-lattice relaxation time of the silicone oil used in phantom fillers.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.